Encoding, transmitting, and decoding video signals is a processor and bandwith intensive process. Typically, analog video must be converted into a digital form, and transmitted as a bitstream over a suitable communication network. When the bitstream arrives at the receiving location, the video data are converted back to a viewable form by decoding. Due to bandwidth constraints of communication channels, video data are often compressed prior to transmission on a communication channel.
One compression technique that takes into account the variable bandwidth availability of communication channels is known as progressive encoding. While any data set that supports lossy compression can be progressively encoded, it is particularly useful for still and video images. Instead of slowly building an accurate image in a single pass, a progressively encoded image quickly provides a crude approximation of the final image, and as time and bandwidth permits, refines the image in later passes. For example, video data can be divided into a “base layer” and one or more “enhancement layers” prior to transmission. The base layer includes a rough version of the video sequence and may be transmitted using comparatively little bandwidth. Typically, the enhancement layers are transmitted at the same time as the base layer, and recombined at the receiving end with the base layer during the decoding process. The enhancement layers provide correction to the base layer, permitting video quality improvement. As will be appreciated, transmitting more enhancement layers produces better output video, but requires more bandwidth. While progressive encoding eases bandwidth constraints, it requires substantially more processor time for encoding and decoding the video signal.